PDRs4All

V. Modelling the dust evolution across the illuminated edge of the Orion Bar^★

¹ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Orsay, France
e-mail: meriem.el-yajouri@universite-paris-saclay.fr
² Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
³ Department of Physics, University of Helsinki, Finland
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden
⁵ Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, Orsay, France
⁶ Observatoire de Paris, PSL University, Sorbonne Université, LERMA, Paris, France
⁷ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Jena, Germany
⁸ Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo, Japan
⁹ Department of Physics & Astronomy, The University of Western Ontario, London, Canada
¹⁰ Institute for Earth and Space Exploration, The University of Western Ontario, London, Canada
¹¹ Carl Sagan Center, SETI Institute, Mountain View, CA, USA
¹² Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹³ ACRI-ST, Centre d’Études et de Recherche de Grasse (CERGA), France
¹⁴ INCLASS Common Laboratory, Grasse, France
¹⁵ DST INSPIRE School of Physics, University of Hyderabad, Hyderabad, India
¹⁶ Space Telescope Science Institute, Baltimore, MD, USA
¹⁷ Sterrenkundig Observatorium, Universiteit Gent, Gent, Belgium
¹⁸ Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia
¹⁹ AIM, CEA, CNRS, Université Paris-Saclay, Université ParisDiderot, Sorbonne Paris-Cité, Gif-sur-Yvette, France
²⁰ California Institute of Technology, IPAC, Pasadena, CA, USA
²¹ NASA Ames Research Center, Moffett Field, CA, USA
²² Physikalischer Verein – Gesellschaft für Bildung und Wissenschaft, Frankfurt am Main, Germany
²³ Institut für Angewandte Physik, Goethe-Universität Frankfurt, Frankfurt am Main, Germany

Received: 24 November 2023
Accepted: 21 December 2023

Abstract

Context. Interstellar dust particles, in particular carbonaceous nano-grains (like polycyclic aromatic hydrocarbons, fullerenes, and amorphous hydrogenated carbon), are critical players for the composition, energy budget, and dynamics of the interstellar medium (ISM). The dust properties, specifically the composition and size of dust grains are not static; instead, they exhibit considerable evolution triggered by variations in local physical conditions such as the density and gas temperature within the ISM, as is the case in photon-dominated regions (PDRs). The evolution of dust and its impact on the local physical and chemical conditions is thus a key question for understanding the first stages of star formation.

Aims. From the extensive spectral and imaging data of the JWST PDRs4All program, we study the emission of dust grains within the Orion Bar – a well-known, highly far-UV (FUV)-irradiated PDR situated at the intersection between cold, dense molecular clouds, and warm ionized regions. The Orion Bar because of its edge-on geometry provides an exceptional benchmark for characterizing dust evolution and the associated driving processes under varying physical conditions. Our goal is to constrain the local properties of dust by comparing its emission to models. Taking advantage of the recent JWST data, in particular the spectroscopy of dust emission, we identify new constraints on dust and further previous works of dust modelling.

Methods. To characterize interstellar dust across the Orion Bar, we follow its emission as traced by JWST NIRCam (at 3.35 and 4.8 μm) and MIRI (at 7.7, 11.3, 15.0, and 25.5 μm) broad band images, along with NIRSpec and MRS spectroscopic observations. First, we constrain the minimum size and hydrogen content of carbon nano-grains from a comparison between the observed dust emission spectra and the predictions of the Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS) coupled to the numerical code DustEM. Using this dust model, we then perform 3D radiative transfer simulations of dust emission with the SOC code (Scattering with OpenCL) and compare to data obtained along well chosen profiles across the Orion Bar.

Results. The JWST data allows us, for the first time, to spatially resolve the steep variation of dust emission at the illuminated edge of the Orion Bar PDR. By considering a dust model with carbonaceous nano-grains and submicronic coated silicate grains, we derive unprecedented constraints on the properties of across the Orion Bar. To explain the observed emission profiles with our simulations, we find that the nano-grains must be strongly depleted with an abundance (relative to the gas) 15 times less than in the diffuse ISM. The NIRSpec and MRS spectroscopic observations reveal variations in the hydrogenation of the carbon nano-grains. The lowest hydrogenation levels are found in the vicinity of the illuminating stars suggesting photo-processing while more hydrogenated nano-grains are found in the cold and dense molecular region, potentially indicative of larger grains.